Fiber-reinforced composite materials having resins reinforced by fibers are widely used in such fields as aircraft, motor vehicles and architecture because of their excellent lightweight and high strength properties.
For obtaining a member made of a fiber-reinforced composite material having a higher strength, it is preferred that the reinforcing fibers coexist with a resin as continuous fibers having no fiber ends therein.
However, continuous fibers are unlikely to be deformed or to migrate when the composite material containing them is molded. For this reason, there can be cases where it is difficult to produce members having, for example, greatly rugged shapes or complicated shapes by molding. In such cases, when a woven fabric composed of continuous fibers is used, it is practiced to form rifts partially in the woven fabric or to remove a portion of the woven fabric, for cutting fibers to have adequate lengths, thereby assuring a degree of freedom in the deformation and mobility of the woven fabric.
In this case, even though the fiber-reinforced composite material uses continuous fibers for reinforcing, the composite material contains the ends of reinforcing fibers therein. As described later, stress concentration occurs near the ends of reinforcing fibers, and can cause the member made from the composite material to be fractured from near the ends. If a molded composite material is fractured at or near fiber ends contained therein, the strength of the composite material becomes lower than the expected strength.
On the other hand, in the case where a member having a more complicated shape is going to be produced, discontinuous fibers having a fiber length of several millimeters to tens of millimeters (usually called short fibers) are used as the reinforcing fibers, to facilitate the mobility and deformation of the reinforcing fibers when the composite material is produced by molding. The obtained composite material is called a short fiber-reinforced composite material.
A typical short fiber-reinforced composite material is called SMC (sheet molding compound). SMC is a sheet-like intermediate base material produced by arranging reinforcing fibers cut to a length of about 10 mm to about 30 mm in one direction or at random on a resin film used as a matrix resin, and further overlaying another resin film on them. The base sheet is placed in a mold installed in a press, pressurized and thermally cured for molding into a bonnet of a motor vehicle, or a housing member such as a bathtub. SMC is used when a member having a complicated shape is produced by means of molding.
However, the short fiber-reinforced composite material has such disadvantages that its strength is lower than one half of the strength of a continuous fiber-reinforced composite material and that its weight is heavier than that of the continuous fiber-reinforced composite material.
The reason why a short fiber-reinforced composite material has a strength lower than that of a continuous fiber-reinforced composite material is that the resin (matrix resin) existing near the fiber ends is fractured. In this case, the reinforcing fibers per se are not fractured, but pulled out from the matrix resin, causing the entire member to be fractured. It is explained in “Composite Material Engineering” edited by Takeshi Hayashi, Union of Japanese Scientists and Engineers, pages 35–36 that in the case where a tensile force acts on a composite material, stress concentration occurs in the matrix resin portions positioned near the ends of reinforcing fibers.
Also in the case where a member is used for a long period of time, if external forces repetitively act on the fiber-reinforced composite material having fiber ends therein, it can happen that the matrix resin is cracked near the fiber ends of the reinforcing fibers, or the matrix resin is separated from the reinforcing fibers, and that the cracking or separation propagates to result in the fracture of the entire member.
As described above, a fiber-reinforced composite material having fiber ends has a problem that since it is fractured from near the fiber ends, the intrinsic properties of the reinforcing fibers cannot be sufficiently transferred.
In view of the problems as described above, it would be advantageous to provide a lightweight high-strength fiber-reinforced composite material though it contains fiber ends therein, and also to provide a production method thereof.